In the industry, in particular the automotive industry, there is an increasing trend towards the utilization of mixed materials for the construction of vehicle bodies for weight and/or cost reasons. Typically, materials of different tensile strength, different melting temperatures and/or different yield strengths and/or different moduli of elasticity are combined with each other. The challenge consists in cohesively connecting or joining the different materials such that large forces and bending moments can be transmitted. For weight reasons, the sheet metal thicknesses furthermore should be optimized corresponding to their strength values, whereby for example steel has a smaller sheet metal thickness than aluminum.
In the prior art, such joints previously have been made from different materials by lap welding, as with lap joints good strength characteristics can be achieved. As regards the lap joints, it was found to be disadvantageous, however, that a thin gap is formed at the joint, into which moisture can penetrate. Due to the penetrating moisture and the different electric potentials of the joint partners, a corrosion cell can form, whereby the welded joint or the welded component can corrode. In addition, such joints cannot be used completely in the visible part of a product, in particular of a vehicle, as a welding seam or an overlap is to be seen, which is perceived as disturbing with regard to the appearance.
Alternatively, it is known from the prior art that two materials can be butt-welded by applying a friction stir welding method. This welding technique among other things offers the advantage that no welding seam is visible, which is perceived as disturbing. Furthermore, by means of the friction stir welding joint in contrast to the lap welding joint higher fatigue strengths can be achieved, as there is no overlap region in which notch effects occur. With the known method, however, only materials of equal material thickness can be connected with each other for different types of joint, so that the resulting strength values meet the requirements. When different materials of different material thickness would be connected with each other at their butt ends, the forces and bending moments transmittable by the joint would correspond to the strength of the material with lower strength based on the cross-section of the thinner material piece. This is due to the fact that in such joints the cross-section of the thinner material piece at the joining line is combined with the strength properties of the material with lower strength, so that the transmittable forces of the joint always are smaller than the transmittable forces of the respective material pieces.
DE 1 901 281 U describes a classical welding method in the case of a lap joint in the region of a boiler bottom, wherein the thin bottom is backed up with a sheet metal at its edge. Both are welded to the thicker jacket wall of the boiler.
In MAG welding the problem exists that a thin sheet metal can burn away very easily due to the extreme input of heat. In this case, lap welding can be made safer by bending over the edge of the sheet metal, as is proposed by DE 10 2010 004 283 A1.
Friction stir welding methods, in which two sheets of equal materials, but of different thickness, are welded to each other, are described in DE 699 33 978 T1 and JP H 10-193 143 A. These documents propose to build-up weld onto the thinner sheet metal or to glue on a further thin sheet metal of the same material.
A friction stir welding method also is shown in JP 2010-036 230 A, in which two sheets of different materials, but of the some thickness, are stir-welded by a tool which has a conically tapered pin.
It therefore is the object of the invention to create a cost-optimized method for butt welding, with which two material pieces of different materials are welded to each other, wherein the strength characteristics of the joint are improved.